Explain how to customize header background beyond just background color

[brentvatne]

Linear gradient, translucent, transparent, etc..

react-navigation/react-navigation#716

[brentvatne]

this is possible using headerBackground and headerTranslucent (both in stack reference) and you can see an example of it in https://github.com/react-navigation/react-navigation/blob/828e7f2d437cab81e9a33584a3d548203169f4de/examples/NavigationPlayground/js/StackWithTranslucentHeader.js

we should document the use case explicitly nonetheless